This proposal is for a study to examine the relationships of diet, nutritional supplement use, and obesity with the risk of prostate cancer. We propose to add a comprehensive dietary assessment component to the Prostate Cancer Prevention Trial (PCH), a trial of the drug finasteride for primary prevention of prostate cancer. The PCPT is recruiting 18,000 men, aged 55 to 70 with no history of cancer or evidence of significant prostatic disease, into a placebo-controlled, double-blinded randomized trial. At seven years post-randomization, all PCPT participants will undergo prostate biopsy to determine the absence or presence of prostate cancer; all tumors will be staged histopathologically and all tissue will be banked and available for additional analyses. We propose to add the following three sets of measures to the PCPT at the one-year post- randomization visits: (1) usual dietary intake of macro- and micro nutrients, using a food frequency questionnaire; (2) nutritional supplement use, using a self-administered questionnaire; and (3) adiposity, including distribution of body fat, assessed as weight, height, and the ratios of waist to hip and waist to thigh circumferences. Principal study results will be based on analyses of the control group only, specifically the 5400 control group men who are expected to remain active through the end of the study. We will estimate the relative risk of prostate cancer using logistic regression models, in which disease outcome at seven years will be defined as: (l) any biopsy-proven cancer; and (2) tumors with a histologically advanced grade (e.g., Gleason score 4+). The independent variable level (categorized) of nutrient intake, and covariates will include potential confounding factors (e.g., total energy intake, age, and race). Additional analyses will incorporate data from the intervention group to assess: (1) the relationship of diet with prostate cancer in persons treated with finasteride; and (2) the effect modification by diet of the relationship between finasteride and prostate cancer. We will validate dietary and nutritional supplement assessment instruments in a substudy of participants who will complete six, 24-hour dietary recalls over a one year period. This proposal is for the first four years of an 11 year study, and includes the first dietary and anthropometric assessments and the validity substudy. At the end of this four year study we will process and make ready for analysis all data from the first dietary and anthropometric assessments. We will give these data, along with a detailed analysis plan, to the Southwest Oncology Group Statistical Coordinating Center. Thus, though we plan to submit competing continuation proposals to fund a second dietary assessment and data analysis, we have made plans to assure the analysis of data from this proposal at the conclusion of the PCPT.